Some wavelength-division-multiplexing-passive-optical-networks (WDM PONs) require precise wavelength alignment between the wavelengths of the signal from a transmitter in a central office to a device in a remote site distributing that signal to a subscriber. In a passive-optical-network, a remote node containing the signal-distributing device is typically located outdoors without any electrical power supply. The transmission band of wavelengths of the outdoor signal-distributing device can change according to the variation of the external temperature. Misalignment of the wavelength between the transmitted signal and the operating wavelength of the device distributing the signal introduces extra insertion loss in the signal.
A possible way to minimize the misalignment can be to use a narrow-linewidth distributed feedback laser diode (DFB LD) that essentially always falls within the shifting bandwidths of the multiplexers as an optical transmitter to satisfy the wavelength alignment condition. However, this arrangement may not be an economic solution because of the high price of each accurately stabilized DFB LD.
Some PONS also use optical transmitters with a high bit rate and an adequate amount of gain to support high bit rate. Some passive optical network may use a broadband light emitting diode (LED) as an optical transmitter. However, the modulation bandwidth of the LED can be narrow, thereby, making it difficult to send data at a high bit rate. Moreover, long-distance transmission in a passive optical network can be difficult with an LED due to the inherent weak power output from an LED.
Some attempts have been made to inject a signal into a laser. However, standard laser chip lengths of 300 microns or so may cause insufficient gain due to no overlap between the bandwidth of an injected signal and a cavity mode of the laser. Also, some standard lasers may not generate suitable gain to support adequate signal quality over a range of operating frequencies when taking in consideration manufacturing tolerances and temperature drift.